The production of decorative surfaced panels designed for such applications as furniture and vertical surfaces where exceptionally high abrasion resistance is not required has increased enormously over the past decade. These panels comprise a single sheet of melamine/formaldehyde resin impregnated decorative paper which is bonded under heat and pressure to a substrate, usually particleboard, of about one-quarter to about one inch in thickness. These products, because they are produced at low pressures, i.e., about 300 psi, and at very short cure cycles, i.e., 2-3 minutes, are relatively inexpensive and have a good appearance and stain resistance.
Abrasion resistance thereof is, however, often poor and attempts have been made to improve the property by providing a layer of clear, unfilled melamine/formaldehyde resin on top of the decorative sheet. While these panels have proven very successful in that the abrasion resistance is adequately elevated, they deteriorate when subjected to humidity conditions encountered in normal use.
This deterioration manifests itself as surface cracks in the panel after it is subjected to low humidity. The cracks are believed to be a result of the dimensional instability of the melamine/formaldehyde resin. These resins undergo dimensional changes owing (1) to loss of water during curing, (2) to cooling after release from the panel press and (3) to loss or gain of water during subsequent exposure to the environment. The dimensional changes are often enough to strain the resin to failure, thus forming cracks. The decorative cellulosic sheet aids in the resistance of the panel to cracking, but its effectiveness is limited by the need for a resin-rich surface to impart abrasion resistance.
Known additives which generally have been added to melamine/formaldehyde resins (such as sucrose, dipentaerythritol sebacate etc.) so as to react with the resin and reduce the tightness of cross-linking usually associated with brittleness do not prevent cracking to a satisfactory degree.
In application Ser. No. 767,721, filed of even date herewith by Huffman, Casey and Thomas, there is described a melamine/formaldehyde resin formulation which can be carried as a transparent film on top of a decorative print sheet in decorative panels to thereby confer abrasion resistance thereto, which will not crack when the panel is subjected to low humidity and will still retain the desirable properties of melamine/formaldehyde resins per se, including transparency and resistance to heat and staining.
The formulation of said application comprises a mixture of a melamine/formaldehyde resin and an ethylene/vinyl chloride copolymer containing amide groups or a polyether based, polyurethane resin containing carboxyl groups. Although this formulation, when impregnated into a fibrous sheet and used to produce a decorative panel, solves the problem of craze and crack formation in the panel at low humidity, it possesses one disadvantage which is especially undesirable in panels which have an intricate printed decorative design on the top surface thereof. The disadvantage manifests itself as a haze in the surface which is not unattractive when solidly colored panels are produced but presents serious aesthetic problems when wood grain or other complex designs are used.